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EP-4444547-B1 - HERRINGBONE MICROSTRUCTURE SURFACE PATTERN FOR FLEXOGRAPHIC PRINTING PLATES

EP4444547B1EP 4444547 B1EP4444547 B1EP 4444547B1EP-4444547-B1

Inventors

  • SIEVERS, WOLFGANG

Dates

Publication Date
20260513
Application Date
20221207

Claims (15)

  1. A process for creating a printing plate for printing ink on a substrate, the printing plate having a printing surface for receiving ink, the process comprising: defining an area of an image intended to print with ink; applying a microstructure surface screen pattern to the defined area, the microstructure surface screen pattern comprising a plurality of rows each having a plurality of diagonal oriented elevated line segments having orientations alternating between a positive angle in one row and a negative angle in an adjacent row, wherein each line segment has a first end (101; 201) aligned with a middle portion (111; 211) of and spaced apart from a first line (100) of a first adjacent row and a second end (102; 202) aligned with a middle portion (122; 222) of and spaced apart from a second line (200) of a second adjacent row.
  2. The process of claim 1, wherein the first end (101; 201) of each line segment in the microstructure surface screen pattern is aligned on center with the first line (100) of the first adjacent row and the second end (102; 202) is aligned on center with the second line (200) of the second adjacent row, and/or wherein each line segment in each row in the microstructure surface screen pattern is oriented perpendicular to adjacent line segments in adjacent rows, and/or wherein the microstructure surface screen pattern includes each elevated line segment having an orientation selected from the group consisting of plus or minus 45-degrees and plus or minus 135-degrees with respect to the rows.
  3. The process of any one of the foregoing claims, wherein the microstructure surface screen pattern includes each of the plurality of elevated line segments having a same number (M) of adjacent touching pixels in total and a same maximum number (T) of pixels arranged in a non-diagonal direction, wherein particularly M is in the range of 3 to 5 and T = 1, or wherein M = 6 and T = 2.
  4. The process of any one of claim 1-3, wherein the microstructure surface screen pattern has a first row of first elevated line segments, a second row of second elevated line segments, and a third row of third elevated line segments, the second row disposed adjacent to and between the first row and the third row, the first elevated line segments and the third elevated line segments oriented in a first direction, and the second row of elevated line segments oriented in a second direction different than the first direction, wherein at least some the first elevated line segments are aligned on a same diagonal line with at least some of the third elevated line segments, or wherein none of the first elevated line segments align on a same diagonal line with any third elevated line segments.
  5. A printing plate comprising a product of the process of any one of claims 1-4.
  6. A printing plate for printing ink on a substrate, the printing plate having a printing surface for receiving ink, the plate having a defined image area intended to print with ink and a microstructure surface screen pattern in the defined area forming the printing surface of the printing plate, the microstructure surface screen pattern comprising a plurality of rows each having a plurality of diagonal oriented elevated line segments having orientations alternating between a positive angle in one row and a negative angle in an adjacent row, wherein each line segment has a first end (101; 201) aligned with a middle portion (111; 211) of and spaced apart from a first line (100) of a first adjacent row and a second end (102; 202) aligned with a middle portion (122; 222) of and spaced apart from a second line (200) of a second adjacent row.
  7. The printing plate of claims 5 or 6, wherein the printing plate comprises an elastomeric printing plate, or wherein the printing plate comprises a photocured monomer or polymer, wherein optionally the microstructure surface screen pattern on the printing plate is a pattern formed by exposing the photocured monomer or polymer to the microstructure surface pattern through a masking layer, or the microstructure surface screen pattern on the printing plate is a pattern formed by embossing the photocured monomer or polymer with the microstructure surface pattern.
  8. A computer implemented method of creating a bitmap for creating a printing plate for printing ink on a substrate, the method comprising: a) providing an image file comprising information defining areas of the plate intended to print with ink; b) providing a microstructure surface screen pattern comprising a plurality of rows each having a plurality of diagonal oriented line segments having orientations alternating between a positive angle in one row and a negative angle in an adjacent row, wherein each line segment has a first end (101; 201) aligned with a middle portion (111; 211) of and spaced apart from a first line (100) of a first adjacent row and a second end (102; 202) aligned with a middle portion (122; 222) of and spaced apart from a second line (200) of a second adjacent row; c) superimposing the microstructure surface screen pattern over the image file.
  9. A computer readable memory media product embodying non-transitory machine readable instructions corresponding to a bitmap produced by the method of claim 8.
  10. A computer readable memory medium embodying non-transitory machine readable instructions for causing an imager to create an image on a mask or film for creating a printing plate, the instructions including information in the form of a bitmap file formed by the process of: providing an image file comprising information defining areas of the plate intended to print with ink; providing a microstructure surface screen pattern comprising a plurality of rows each having a plurality of diagonal oriented line segments having orientations alternating between a positive angle in one row and a negative angle in an adjacent row, wherein each line segment has a first end (101; 201) aligned with a middle portion (111; 211) of and spaced apart from a first line (100) of a first adjacent row and a second end (102; 202) aligned with a middle portion (122; 222) of and spaced apart from a second line (200) of a second adjacent row; and superimposing the microstructure surface screen pattern over the image file.
  11. A tool for use in making a printing plate having a microstructured printing surface for receiving ink, the tool comprising a pattern applied to the tool, the pattern comprising a plurality of rows each having a plurality of diagonal oriented elevated line segments having orientations alternating between a positive angle in one row and a negative angle in an adjacent row, wherein each line segment has a first end (101; 201) aligned with a middle portion (111; 211) of and spaced apart from a first line (100) of a first adjacent row and a second end (102; 202) aligned with a middle portion (122; 222) of and spaced apart from a second line (200) of a second adjacent row.
  12. The tool of claim 11, wherein the tool is a mask or a film for use as a masking layer during an exposure step.
  13. The tool of claim 11, wherein the tool is an embossing cylinder, wherein optionally, the embossing cylinder comprises metal, ceramic, or a combination thereof.
  14. A process of making a printing plate having a microstructured printing surface for receiving ink, the process comprising: providing the tool comprising the mask or film of claim 12 disposed on a photocurable printing plate precursor; exposing the photocurable printing plate precursor to actinic radiation through the mask or film; and developing the printing plate from the plate precursor.
  15. A process of making a printing plate having a microstructured printing surface for receiving ink, the process comprising: a) providing the tool comprising the embossing cylinder of claim 13; b) providing a mask defining an image; c) providing a photocurable printing plate precursor; d) exposing the photocurable printing plate precursor to actinic radiation through the mask or film; e) developing the printing plate from the plate precursor to form elevated ink receiving areas; and f) embossing the microstructured printing surface on the elevated ink receiving areas using the embossing cylinder.

Description

BACKGROUND OF THE INVENTION Flexographic printing using photopolymer printing plates has seen continuous improvement over the last decades. Around 2010, high intensity UV LEDs enabled creation of flat top dots, which have enabled further improvements of the ink transfer. While flat top dots were initially used to improve highlight dots in print, it was soon discovered they also could be used for creation of microstructures on the printing surface of a plate. Micro-structured surface patterns can improve ink splitting during print, which can result in higher Solid Ink Density, lead to a more homogeneous ink film on the print substrate, less pinholes in the ink film, or combinations thereof. One application for surface patterns include improving the print properties of process colors, by improving the definition of vignettes, smoothening tonal transitions, reducing dot gain, dot bridging and trailing edge voids, increasing Solid Ink Density (SID), and reducing pinholes in the ink film. Another application for surface patterns includes improving ink transfer for spot colors to provide higher SID, reduced pinholes and less mottling. Spot colors are used, for example, to create a base for process colors on a transparent substrate (e.g. a white underprint), to produce color-accurate brand colors (e.g., the distinctive red color used by Coca Cola®), or as a sealing layer overprinted over process colors (e.g. on a transparent substrate). Several patterns have been evaluated for this purpose. Most successful have been line patterns. In the field of flexographic printing with photopolymer plates micro-structured printing surfaces, also called surface patterns or micro screens, are well known. EP1557279 (Dewitte) discloses a so-called, "groovy pattern" comprising parallel lines. Groovy surface patterns comprise parallel lines oriented +45 or -45° with respect to imaging and printing direction. The lines of these patterns were typically several image pixels wide. These surface patterns were already applied years before the invention of Flat Top Dots. Fig. 1A shows an example of such surface pattern which is available in the screen libraries of Esko raster image processors (RIPs) as the "MG25" pattern. The principle was mainly applied to digital photopolymer printing plates using a laser to write image information into a Laser ablateable mask (LAMs) on top of the polymer plate, but it also worked with analog photopolymer printing plates using film for transferring image information to the polymer plate. These patterns can also be directly engraved as three dimensional structures into elastomeric flexographic printing plates by laser ablation. This pattern may have some orientation sensitivity with respect to the imaging direction in some embodiments. It is also known that the ink transfer of solid areas may be improved by overlaying high-percentage, high-line-count screens over the solid printing areas. For example, using a 516 LPI 73% screen instead of a 100% solid area may improve SID. Such a screen is shown in Fig. 1B. this surface pattern is available in the screen libraries of Esko RIPs as the "MC16P" pattern. This pattern was discovered and used early in the field of flexography when printers found that SID always dropped slightly when the raster screen turned from 99 to 100 %. After the advent of flat top dots, it was discovered that small dots generated by single pixel mask openings, imaged with "boosted" laser power, led to an even greater improvement in SID compared to the previous methods. Fig. 1E shows one example of a single pixel screen, available in the Esko RIP screen library as "MCWSI pattern n." "Boosting" refers to increasing the laser power for a single pixel in the imaging direction relative to the power typically used for connected rows of pixels in the imaging direction. Flat top dot UV exposure of polymer plates allows creation of dots comprising only a single pixel, which in turn allows relatively higher line counts for the screens over a solid area. This concept is described in more detail in WO2017203034A1. This technology as described therein led to modified groovy patterns comprising single pixel lines as shown in Figs. 1C and 1D. These Patterns are named "MG45" and "MG34," respectively, in the Esko RIP screen library. It has been found that, especially for spot colors, where higher ink volumes need to be transferred to the print substrate, micro screen patterns comprising line structures can perform better than true single pixel structures, as is described in more detail in application US 16/950,361. Also, in narrow web print applications for label printing, line patterns are often preferred over true single pixel structures such as those depicted in FIG. 1G ("MCWSI") and Fig 1H ("DDWSI"). Accordingly, there remains a need in the art for micro-structure patterns that having an optimum combination of attributes for improving ink distribution on the print substrate and reducing pinholes in th